Metals chelation mechanisms

Studies in the photoinitiation of polymerization by transition metal chelates probably stem from the original observations of Bamford and Ferrar [33]. These workers have shown that Mn(III) tris-(acety]acetonate) (Mn(a-cac)3) and Mn (III) tris-(l,l,l-trifluoroacetyl acetonate) (Mn(facac)3) can photosensitize the free radical polymerization of MMA and styrene (in bulk and in solution) when irradiated with light of A = 365 at 25°C and also abstract hydrogen atom from hydrocarbon solvents in the absence of monomer. The initiation of polymerization is not dependant on the nature of the monomer and the rate of photodecomposition of Mn(acac)3 exceeds the rate of initiation and the initiation species is the acac radical. The mechanism shown in Scheme (14) is proposed according to the kinetics and spectral observations ... 

In addition to the above mechanism, the metal chelates can influence the process of quenching through electron-transfer ... 

Kinetics and mechanism of metal chelation processes via solvent extraction techniques. H. Freiser, Acc. Chem. Res., 1984,17,126-131 (40). 

Alt H, Binder H, Sandstete G (1973) Mechanism of the electrocatalytic reduction of oxygen on metal chelates. J Catal 28 8-19... 

In the rhizosphere, microorganisms utilize either organic acids or phytosiderophores to transport iron or produce their own low-molecular-weight metal chelators, called siderophores. There are a wide variety of siderophores in nature and some of them have now been identified and chemically purified (54). Pre.sently, three general mechanisms are recognized for utilization of these compounds by microorganisms. These include a shuttle mechanism in which chelators deliver iron to a reductase on the cell surface, direct uptake of metallated siderophores with destructive hydrolysis of the chelator inside the cell, and direct uptake followed by reductive removal of iron and resecretion of the chelator (for reviews, see Refs. 29 and 54). 

In a bidentate ligand system, three molecules of a dye containing either a terminal salicylic acid unit (as in 5.2) or an o-nitrosonaphthol residue are able to chelate simultaneously with a trivalent metal ion of CN6, such as chromium (III) or iron(III), to form a 1 3 metal-dye complex (as in 5.8). Historically, the most important bidentate ligand system was alizarin (5.1). It has been suggested that both hydroxy groups and the keto group in the peri position are all involved with the metal atom in the chelation mechanism. 

Anthocyanins have the potential to moderate the total oxidative load via three mechanisms. First, they can chelate to copper and iron, thereby decreasing the possibility of hydroxyl radical production from Haber-Weiss reactions. These chelates might also protect other low molecular weight antioxidants (LMWAs), such as ascorbate and a-tocopherol, from autoxidation by transition metals.Anthocyanin-transition metal chelation has been demonstrated in vitro many times,but is unlikely to feature significantly in planta. 

Simionescu and coworkers (36) also carried out for the first time mechanical reactions leading to complex metal chelates with macromolecular ligands. Fragments formed from the mechanical condensation of polyethylene tereph-thalate with ethylene diamine serve as ligands for ferric chloride. The reaction was performed in a vibromill (amplitude 4 mm, 1475 rpm at 18° C). 

Subsequently, an amine, R NH2, would react with the monochelated species in the manner suggested in the Verter-Frost mechanism. It seems likely that the driving force for the reaction would be the formation of the more stable metal-chelate compound. The equilibrium could also be shifted by loss of volatile amine—e.g., ammonia. 

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